Positive electrode material for lithium secondary cell and process for producing the same

ABSTRACT

A positive electrode material for a lithium secondary battery for high voltage high capacity use exhibiting high cycle durability and high safety. The positive electrode material is composed of particles having a composition represented by the general formula: Li a Co b Mg c A d O e F f  (A is the group 6 transition element or the group 14 element, 0.90≦a≦1.10, 0.97≦b≦1.00, 0.0001≦c≦0.03, 0.0001≦d≦0.03, 1.98≦e≦2.02, 0≦f≦0.02 and 0.0001≦c+d≦0.03), and magnesium, the element A and fluorine exist uniformly in the vicinity of the surfaces of the particles.

RELATED APPLICATIONS

The present application is based on International Application No.PCT/JP2004/011748 filed Aug. 16, 2004, and claims priority from,Japanese Application Number 2003-295171 filed Aug. 19, 2003,respectively, the disclosure of which is hereby incorporated byreference herein in its entirety.

TECHNICAL FIELD

The present invention relates to a positive electrode material for alithium secondary battery that exerts high-capacity and high-cyclecharacteristics particularly in the use at high voltages, and a methodfor manufacturing the same.

BACKGROUND ART

In recent years, with the progress of various portable and cordlesselectronic appliances, demands for small and light nonaqueouselectrolyte secondary batteries having high energy density have beenincreased, and the development of a positive electrode material for anonaqueous electrolyte secondary battery has been desired than everbefore.

As the material of a positive electrode for a nonaqueous electrolytesecondary battery, LiCoO₂, LiNiO₂, LiMn₂O₄ or the like has been used,and especially, LiCoO₂ is used in a large quantity from the aspects ofsafety, capacity and the like. In this material, lithium in the crystallattice is released into an electrolyte solution as lithium ions withcharging, and the lithium ions are reversibly inserted into the crystallattice from the electrolyte solution with discharging, to exert thefunctions as a positive electrode active material.

Theoretically, one lithium ion can be released from and inserted intoone LiCoO₂ lattice. Actually, however, if most of lithium is released orinserted, LiCoO₂ is significantly deteriorated causing damage especiallyto cyclic performance. Therefore, in the present situation, about 0.55lithium ions are released from and inserted into one LiCoO₂, and at thistime, the capacity of only about 150 mAh is used for 1 g of LiCoO₂.

Although the expansion of the capacity can be expected by releasing andinserting larger quantities of lithium ions, if more lithium ions thanthe present quantities are released and inserted, there are problemsthat intense deterioration of LiCoO₂ occurs and sufficient cyclicperformance cannot be secured by the phase transition of the LiCoO₂crystal lattice, accompanying damage of particles and crystal lattice,or the elution of cobalt ions from the crystal lattice.

Although there has been an attempt to improve the cyclic durability at4.5 V by doping 5% by weight of zirconium into LiCoO₂, the initialcapacity lowers significantly, and cyclic durability is also notsatisfactory (refer to Non-Patent Document 1 described below).

[Non-Patent Document 1]: Z. Chen, J. R. Dahn, 11th International Meetingof Lithium Battery Jun. 23-28, 2002, Monterey, USA, Abstract No. 266

DISCLOSURE OF THE INVENTION PROBLEMS TO BE SOLVED BY THE INVENTION

Therefore, an object of the present invention is to provide a positiveelectrode material for a lithium secondary battery for high voltagesthat excels in the prevention of deterioration due to high voltages, andexcels in high capacity, high safety and cyclic durability.

MEANS FOR SOLVING THE PROBLEMS

As a result of repeated keen studies to solve the above-describedproblems, the present inventors found that by simultaneously adding aspecific amount of magnesium (Mg) and a specific metal element to aparticulate positive electrode active material mainly composed oflithium cobaltate for a lithium secondary battery, or by furthersimultaneously added fluorine, favorable cyclic performance could beachieved even in a high-voltage region, which was conventionally deemedto be overcharged.

In the present invention, high voltages mean voltages wherein a chargingvoltage is 4.4 V or higher with reference to the lithium batteryelectrode. Further, a specific example of the charging voltage is 4.5 V.At this time, the capacity of about 185 to 190 mAh can be used in 1 g ofLoCoO₂, which is equivalent to the release and insertion of about 0.7lithium atoms per LoCoO₂ molecule.

In the present invention, although the mechanism why favorable cyclicperformance is developed is not necessarily clear in a high-voltageregion, it is considered that since magnesium and a specific metalelement are simultaneously added, and these are present on the surfaceof a particle, or a part of these forms a solid solution, these act as asupport of the crystal lattice under a high-voltage condition whereinmost lithium ions are extracted; and these relaxes the strain of thecrystal lattice accompanying phase transition or expansion andshrinkage, and suppress deterioration. At the same time, it isconsidered that since the chance of direct contact of cobalt atoms withthe electrolyte is reduced, and overcharged state locally occurring inparticles is eliminated, deterioration can be suppressed.

Thus, the positive electrode material for a lithium secondary battery ofthe present invention is characterized in that an active material for apositive electrode consists of particle having the compositionrepresented by a general formula, Li_(a)Co_(b)Mg_(c)A_(d)O_(e)F_(f) (Ais the group 6 transition element or the group 14 element, 0.90≦a≦1.10,0.97≦b≦1.00, 0.0001≦c≦0.03, 0.0001≦d≦0.03, 1.98≦e≦2.02, 0≦f≦0.02 and0.0001≦c+d≦0.03) wherein magnesium, the element A, or further fluorineare evenly present in the vicinity of the surface of the particle.

In the present invention, the term “evenly present” includes not onlythe case wherein each of the above-described elements is substantiallyevenly present in the vicinity of the particle surface, but also thecase wherein the quantities of the above-described elements presentbetween particles are substantially identical, and it is sufficient ifeither one is satisfied, and it is especially preferable that both ofthem are satisfied. In other words, it is especially preferable thatquantities of the above-described elements present between particles aresubstantially identical, and the above-described elements are evenlypresent on the surface of a particle.

In the present invention, it is preferable that at least a part ofmagnesium or an element represented by A substitutes cobalt atom in theparticles, and forms a solid solution. It is more preferable that theatomic ratio of magnesium to the element A is 0.10≦c/d≦10.00, and0.0002≦c+d≦0.02.

In the present invention, the element A is selected from group 6transition elements or group 14 elements. It is considered thatmagnesium substitutes mainly a lithium site. The element A is preferablytungsten or silicon.

The present invention provides a positive electrode material for alithium, secondary battery characterized in that the element A istungsten, and no diffraction peaks are observed within the rang of2θ=21±1° in the high-sensitivity X-ray diffraction spectrum using Cu—Kα.If silicon is used for the element A, an identical positive electrodematerial for the lithium second battery can be obtained.

In the present invention, the high-sensitivity X-ray diffractionspectrum means a diffraction spectrum obtained at an acceleratingvoltage of the X-ray tube of 50 kV and an accelerating current of 250mA. Ordinary X-ray diffraction spectrum is obtained at an acceleratingvoltage of about 40 kV and an accelerating current of about 40 mA, whichis difficult to accurately detect a trace amount of impurity phasenoticed by the present invention and significantly affecting batterycharacteristics in a short time while suppressing analysis noise.

Here, the bonding state of the element A, cobalt atoms, lithium atomsand oxygen atoms can be determined by the high-sensitivity X-raydiffraction spectrum. For example, when the element A is tungsten, whichforms a solid solution with cobalt atoms, since no diffraction spectraderived from the single oxide of tungsten (WO₃) are observed, thediffraction spectrum intensity of the single oxide of the element A canbe measured to calculate the solid dissolution quantity of the elementA. The element A substitutes the cobalt site to form a solid solution,the solid dissolution quantity thereof is preferably 60% or more, andmore preferably 75% or more.

The present inventors found that the battery performance was improvedwhen the quantity of the element A present as a single oxide was small.Therefore, the present invention provides a positive electrode materialfor a lithium secondary battery characterized in that the quantity ofthe element A present as a single oxide is 20% or smaller. The quantityof the single oxide of the element A exceeding 20% is not preferablebecause the effect of improving charge-discharge cyclic durability athigh voltages is lowered. The quantity of the single oxide of theelement A is more preferably 10% or smaller.

The present inventors found that the charge-discharge cyclic durabilityat high voltages of the positive electrode material having a specificstructure obtained by selecting tungsten as the element A, allowingmagnesium to co-exist, and manufactured by a specific method wasmarkedly improved.

Here, specifically, it is important in the specific structure that addedtungsten is not present on the surface of the particle of lithiumcobaltate as a single oxide. For this purpose, the present inventorsfound to be particularly preferable that magnesium was added to tungstenin the above-described specific atomic ratios (0.10≦c/d≦10.00, and0.002≦c+d≦0.02.), and lithium cobaltate was formed in the coexistence ofa tungsten compound and a magnesium compound. Specifically, the presentinventors found that the coexistence of magnesium had a significanteffect to raise the reactivity of tungsten. It was also found that thepresence of magnesium had an effect to lower the Co₃O₄ content in theformed lithium cobaltate.

Although the action mechanism for the unique improvement ofcharacteristics obtained by the simultaneous addition of tungsten andmagnesium has not be clarified, it is estimated that a uniform inactivefilm is formed on the surface of lithium cobaltate particles by thesimultaneous addition of tungsten and magnesium, and the decay ofcrystals from the surfaces of the particles accompanying the chargingand discharging of the lithium cobaltate crystal structure can besuppressed.

Although it was described that a specific improvement of characteristicswas observed by the addition of magnesium at the same time when tungstenor silicon was used as the element A, the combination of such elementsis not limited to this combination, but the coexistence of a specificquantity of magnesium in the combination of another element A is alsoeffective to improve the characteristics as the positive electrodematerial for a lithium secondary battery, because it raises thereactivity of the element A and suppresses the formation of a singleoxide.

The present invention also provides a positive electrode material for alithium secondary battery wherein the positive electrode active materialis a secondary particle wherein 10 or more primary particles areaggregated, and the average particle diameter of the secondary particlesare 2 to 20 μm. By using such a secondary particle structure of anaggregated body, the packing density of the active material for theelectrode layer and large current charge-discharge performance can beimproved.

Furthermore, the present invention provides a method for manufacturing apositive electrode material for a lithium secondary batterycharacterized in that an active material for a positive electrodeconsists of particles having the composition represented by a generalformula, Li_(a)Co_(b)Mg_(c)A_(d)O_(e)F_(f) (A is the group 6 transitionelement or the group 14 element, 0.90≦a≦1.10, 0.97≦b≦1.00,0.0001≦c≦0.03, 0.0001≦d≦0.03, 1.98≦e≦2.02, 0≦f≦0.02 and 0.0001≦c+d≦0.03)wherein magnesium, the element A, or further fluorine is evenly presentin the vicinity of the surface of the particles, and a cobalt materialconsisting of particles wherein 10 or more primary particles aggregateto form secondary particles and containing at least either cobaltoxyhydroxide or cobalt hydroxide, lithium carbonate, and a materialcontaining magnesium, the element A or further fluorine are mixed andfired.

EFFECT OF THE INVENTION

According to the present invention, there is provided a positiveelectrode material for a lithium secondary battery having high cyclicdurability and high safety in high-voltage and high-capacity uses usefulfor the lithium secondary battery. The lithium cobaltate positiveelectrode of this invention is useful since it has high charge-dischargecyclic performance not only in a high-voltage use, but also in the casewherein it is used for a positive electrode for a normal lithium-ionbattery of 4.2-V to 4.3-V classes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is X-ray diffraction spectra of the powder of the positiveelectrode material obtained in Example 1 of the present invention; and

FIG. 2 is X-ray diffraction spectra of the powder of the positiveelectrode material obtained in Comparative Example 2.

BEST MODE FOR CARRYING OUT THE INVENTION

In the present invention, the particulate active material of thepositive electrode for a lithium secondary battery has a compositionrepresented by a general formula, Li_(a)Co_(b)Mg_(c)A_(d)O_(e)F_(f). Inthis general formula, a is 0.90 to 1.10, b is 0.97 to 1.00, c is 0.0001to 0.03, d is 0.0001 to 0.03, e is 1.98 to 2.02, f is 0 to 0.02 and c+dis 0.0001 to 0.03. The element A is at least an element that belongs tothe group 6 element or the group 14 element, such as chromium (Cr),molybdenum (Mo), tungsten (W), silicon (Si), tin (Sn) and lead (Pb),among which tungsten and silicon are preferred from the aspect ofperformance.

In the present invention, the shape of the particle of the activematerial for the positive electrode is preferably spherical, and as thesize thereof, the average particle diameter is preferably 2 to 20 μm,more preferably 3 to 15 μm. The average particle diameter smaller than 2μm is not preferable because the formation of a dense electrode layerbecomes difficult; and on the contrary, the average particle diameterthat exceeds 20 μm is not preferable because the formation of a flatsurface of the electrode layer becomes difficult.

The above-described active material for the positive electrode is formedof secondary particles wherein preferably 5 or more particles, morepreferably 10 or more particles are aggregated, and thereby the packingdensity of the active material for the electrode layer can be improved,and large current charge-discharge performance can also be improved.

In the present invention, magnesium, the above-described element A, orfluorine (F) must be substantially evenly present on the surface of theparticle of the particulate active material for the positive electrode.The presence of these atoms inside the particles is not only useless,but also a large quantity of these atoms must be added to be presentinside, and then, since it rather causes the lowering of initialcapacity, the lowering of large current charge-discharge performance orthe like, it is preferable to make the atoms be present on the surfaceof the particle by the small quantity of addition. Among these,magnesium and the element A are present preferably within 100 nm, morepreferably within 30 nm from the surface of the particle.

A part of magnesium and the element A present on the surface of theparticles of the above-described particulate active material for thepositive electrode is preferably a solid solution wherein cobalt atomsinside the particle are substituted. A part of fluorine is preferably asolid solution wherein oxygen atoms inside the particle are substituted.These cases are preferable, because cobalt and oxygen atoms are notexposed on the surface of the particles of the active material for thepositive electrode, and the effect of the added elements is moresignificantly manifested. As a result, the cyclic properties as theactive material for the positive electrode for high voltages can beeffectively improved. The addition of fluorine atoms is preferablebecause it has the effect of improving the safety and cyclic performanceof the battery.

It was found that the atomic ratios of magnesium atoms, element A atomscontained in the particulate active material for the positive electrodeof the present invention, and cobalt atoms (c/b and d/b) were requiredto be 0.0001 to 0.02 each, these must be simultaneously added, and theatomic ratio of the total quantity of magnesium atoms and element Aatoms to cobalt atoms ((c+d)/b) was required to be 0.0001 to 0.02.

Each of such magnesium atom ratio and element A atom ratio smaller than0.0001 is not preferable, because the effect of improving high cyclicperformance is reduced. On the other hand, the atomic ratio of the totalquantity of magnesium atoms and element A atoms exceeding 0.02 is notpreferable, because the initial capacity is significantly lowered.

The atomic ratio of fluorine atoms to cobalt atoms is preferably 0.0001to 0.02, more preferably 0.0005 to 0.008, for the improvement of safetyand cyclic performance. The atomic ratio of fluorine atoms exceedingthese values is not preferable, because the discharge capacity ismarkedly lowered.

Furthermore, it is preferable that the particulate active material for apositive electrode of the present invention has a press density of 2.7to 3.3 g/cm³. The press density lower than 2.7 g/cm³ is not preferable,because the initial volume capacity density of the positive electrode islowered when a positive electrode sheet is formed using the particulateactive material for a positive electrode; and on the contrary, the pressdensity higher than 3.3 g/cm³ is also not preferable, because theinitial weight capacity density of the positive electrode is lowered, orhigh-rate discharging characteristics are lowered. Particularly, thepress density of the particulate active material for a positiveelectrode is preferably 2.9 to 3.2 g/cm³.

In the present invention, since the press density can be raised, it ispreferable to use substantially spherical cobalt oxyhydroxide wherein alarge number of primary particles aggregate to form a secondaryparticle, as the cobalt source. The press density used herein means thevalue obtained from the volume and weight of the powder when the powderis pressed under a pressure of 0.32 t/cm².

In the present invention, the specific surface area of the particulateactive material for a positive electrode is preferably 0.2 to 1.2 m². Ifthe specific surface area is smaller than 0.2 m²/g, the dischargecapacity per initial unit weight lowers; and on the contrary, if itexceeds 1.2 m²/g, the discharge capacity per initial unit weight alsolowers; and an excellent active material for a positive electrode, whichis the object of the present invention, cannot be obtained.Particularly, the specific surface area is preferably 0.3 to 1.0 m²/g.

The methods for manufacturing the particulate active material for apositive electrode of the present invention are not necessarily limited,but various known methods can be used. For example, as a cobalt source,cobalt hydroxide, tricobalt tetraoxide, or cobalt oxyhydroxide is used,and particularly, cobalt oxyhydroxide and cobalt hydroxide arepreferable, because these exert high battery performance. Also as acobalt source, a cobalt material consisting of particles wherein 10 ormore primary particles aggregate to form secondary particles, andcontaining at least either cobalt oxyhydroxide or cobalt hydroxide ispreferable, because high battery performance can be obtained.

As the materials for magnesium and element A, oxides, hydroxides,chlorides, nitrates, organic acid salts, oxyhydroxides, or fluorides areused, and particularly, hydroxides, oxides, or fluorides are preferable,because these exert high battery performance easily. As the material forlithium, lithium carbonate or lithium hydroxide is preferable. As thematerial for fluorine, lithium fluoride, aluminum fluoride, or magnesiumfluoride is preferable.

The particulate active material for a positive electrode according tothe present invention is manufactured by firing the mixture of thesematerials, preferably, the mixture of at least one that can be selectedfrom oxides containing magnesium or element A, or hydroxides containingmagnesium or element A, lithium fluoride, cobalt hydroxide, cobaltoxyhydroxide or cobalt oxide and lithium carbonate in anoxygen-containing atmosphere at 600 to 1050° C., preferably at 850 to1000° C. for 4 to 48 hours, preferably 8 to 20 hours to convert it to acomposite oxide. A favorable battery performance can be obtained when afluoride containing magnesium or element A is used in place of acompound containing magnesium or element A and lithium fluoride.

As the oxygen-containing atmosphere, an oxygen-containing atmospherewherein the oxygen concentration is preferably 10% by volume or higher,more preferably 40% by volume or higher is preferably used. Such acomposite oxide can satisfy the above-described present invention bychanging the kind, the mixed composition and the firing conditions ofthe above-described materials. In the present invention, preliminaryfiring can be performed before the above-described firing. Thepreliminary firing is preferably performed in an oxidizing atmosphere atpreferably 450 to 550° C. for preferably 4 to 20 hours.

The method for manufacturing the active material for a positiveelectrode of the present invention is not limited to the above-describedmethod, but for example, it can be manufactured by synthesizing anactive material for a positive electrode using a metal fluoride, anoxide and/or a hydroxide as materials, and surface-treating using afluorination agent, such as fluorine gas, NF₃, and HF.

The method for obtaining a positive electrode for a lithium secondarybattery from the positive electrode material of the present inventioncan be conducted in accordance with ordinary methods. For example, acombined agent for the positive electrode can be prepared by mixing acarbon-based conductive material, such as acetylene black, graphite andketjen black, and a binder to the powder of the active material for apositive electrode of the present invention. As the binder,polyvinylidenefluoride, polytetrafluoroethylene, polyamide,carboxymethylcellulose, acrylic resins or the like is used. A slurrywherein the above-described combined agent for the positive electrode isdispersed in a dispersant, such as N-methyl pyrrolidone is applied ontothe positive electrode collector, such as an aluminum foil, dried androll-pressed to form an active material layer for a positive electrodeon the positive electrode collector.

Concerning the lithium battery using the positive electrode material ofthe present invention as the positive electrode, a carbonate ester ispreferable as a solvent of the electrolyte solution. Either cyclic orchain carbonate ester can be used. The examples of cyclic carbonateesters include propylene carbonate, ethylene carbonate (EC) and thelike. The examples of chain carbonate esters include dimethyl carbonate,diethyl carbonate (DEC), ethyl methyl carbonate, methyl propylcarbonate, methyl isopropyl carbonate, and the like.

The above-described carbonate esters can be used alone or in combinationof two or more. They can also be used in combination with othersolvents. In some active materials for the negative electrode,discharging characteristics, cyclic durability, and the charge-dischargeeffect can be improved by using a chain carbonate ester and a cycliccarbonate ester in combination. Alternatively, a gel polymer electrolytecan be prepared by adding a vinylidene fluoride-hexafluoropropylenecopolymer (e.g., Kynar manufactured by Atochem) and a vinylidenefluoride-perfluoropropyl vinyl ether copolymer to these organicsolvents, and adding the following solute.

As the solute of the electrolyte solution, any one or more of lithiumsalts containing ClO₄—, CF₃SO₃—, BF₄— PF₆—, AsF₆—, SbF₆—, CF₃CO₂—,(CF₃SO₂)₂N— or the like as anions are preferably used. It is preferablethat the above-described electrolyte solution or polymer electrolyte areobtained by adding a lithium salt to the above-described solvent orsolvent-containing polymer in the concentration of 0.2 to 2.0 mol/L. Ifthe concentration deviates from this range, ion conductivity is lowered,and the electrical conductivity of the electrolyte is lowered. Morepreferably, 0.5 to 1.5 mol/L is selected. As the separator, a porouspolyethylene or porous polypropylene film is used.

The negative electrode active material for a lithium battery using thepositive material of the present invention as the positive electrodethereof is a material that can occlude and release lithium ions.Although materials for forming the negative electrode active materialare not specifically limited, the examples of such materials includemetallic lithium, lithium alloys, carbon materials, oxide based onmetals of groups 14 and 15 in the periodic table, carbon compoundssilicon carbide compounds, silicon oxide compounds, titanium sulfide andboron carbide compounds.

As carbon materials, thermally decomposed organic materials undervarious thermal decomposition conditions, artificial graphite, naturalgraphite, soil graphite, expanded graphite, flake graphite or the likecan be used. As oxides, compounds based on tin oxide can be used. As thenegative electrode collector, copper foil, nickel foil or the like canbe used. The type of the lithium secondary battery using the positiveelectrode material in the present invention is not specifically limited.Sheet type (i.e., film type), folded type, wound cylinders with bottoms,button type or the like are selected for usage.

EXAMPLES Example 1

Predetermined quantities of cobalt oxyhydroxide powder, wherein 50 ormore primary particles were aggregated to form secondary particles, ofan average particle diameter D50 of 10.2 μm, lithium carbonate powder,magnesium hydroxide powder, and tungsten oxide powder were mixed. Afterdry-mixing these four kinds of powders, the mixture was fired in the airat 950° C. for 14 hours. As a result of wet-dissolution of the firedpowder, and the measurement of the contents of cobalt, magnesium,tungsten and lithium by ICP and atomic absorption spectrometry, thecomposition of the powder was LiCo_(0.99)Mg_(0.005)W_(0.005)O₂.

The specific surface area of the fired powder (active material powderfor the positive electrode) measured by a nitrogen adsorption method was0.40 m²/g, and the average particle diameter D50 measured by a laserscattering type particle size distribution analyzer was 14.0 μm. As aresult of XPS analysis of the surface of the powder after firing, astrong signal of Mg2P caused by magnesium and a strong signal of W4Pcaused by tungsten were detected.

When XPS analysis was performed after sputtering the powder for 10minutes, the signals of magnesium and tungsten by XPS were attenuated to10% and 13% of the signals before sputtering, respectively. Thissputtering is equivalent to surface etching of a depth of about 30 nm.It was known from this that magnesium and tungsten were present on thesurface of the particle. As a result of observation through SEM(scanning electron microscope), 10 or more primary particles wereaggregated to form a secondary particle in the obtained active materialpowder for the positive electrode.

By a high-sensitivity X-ray diffraction method using a Model RINT2 500X-ray diffraction apparatus with Cu—Kα manufactured by RigakuCorporation, X-ray diffraction spectra of the powder after firing wereobtained under conditions of an accelerating voltage of 50 kV, andaccelerating current of 250 mA, a scanning rate of 1°/min, a step angleof 0.02°, a divergence slit of 1°, a scattering slit of 1°, a lightreceiving slit of 0.3 mm, and in the presence of monochromatization Theobtained spectrum is shown in FIG. 1. From FIG. 1, no diffractionspectrum was observed within the range of 2 θ=21±1°, and it was foundthat tungsten was not present as a single oxide.

Thus obtained LiCo_(0.99)Mg_(0.005)W_(0.005)O₂ powder, acetylene black,and polytetrafluoroethylene powder were mixed in a weight ratio of80/16/4, kneaded while adding toluene, and dried to fabricate a positiveelectrode plate of a thickness of 150 μm.

Then, a model sealed battery made from stainless-steel was assembled inan argon glove box using an aluminum foil of a thickness of 20 μm as thepositive electrode collector, using a porous polypropylene of athickness of 25 μm as a separator using a metallic lithium foil of athickness of 500 μm as the negative electrode, using a nickel foil of athickness of 20 μm as the negative electrode collector, and using1MliPF₅/EC+DEC (1:1) as the electrolyte.

This battery was first charged to 4.5 V at a load current of 75 mA pergram of the active material for the positive electrode at 25° C., anddischarged to 2.75 V at a load current of 75 mA per gram of the activematerial for the positive electrode to obtain the initial dischargecapacity. Further, charge-discharge cycle tests were repeated 50 times.

The initial discharge capacity at 25° C., 2.75 to 4.5 V, and a dischargerate of 0.5 C was 190.2 mAh/g, and the average voltage was 4.03 V. Thecapacity retention after 50 charge-discharge cycles was 93.5%.

Another similar battery was fabricated. After this battery was chargedat 4.3 V for 10 hours, the battery was disassembled in an argon glovebox. Thereafter the positive electrode body sheet after being chargedwas taken out, punched to a diameter of 3 mm after washing, sealed in analuminum capsule together with EC, and heated at a rate of 5° C./minwith a scanning differential calorimeter to measure heat generationstarting temperature. As a result, the heat generation startingtemperature of the 4.3-V charged product was 165° C.

Example 2

An active material for a positive electrode was synthesized in the samemanner as in Example 1 except that predetermined quantities of cobaltoxyhydroxide powder, wherein 50 or more primary particles wereaggregated to form secondary particles, of an average particle diameterD50 of 10.7 μm, lithium carbonate powder, magnesium hydroxide powder,tungsten oxide powder, and lithium fluoride powder were mixed, andcomposition analysis, property measurement, and battery performance testwere conducted. As a result, the composition wasLiCo_(0.99)Mg_(0.005)W_(0.005)O_(1.9924)F_(0.0076.)

The specific surface area of the powder after firing measured by anitrogen adsorption method was 0.31 m²/g, and the average particlediameter D50 measured by a laser scattering type particle sizedistribution analyzer was 14.5 μm. Magnesium, tungsten and fluorine werepresent on the surface. As a result of observation through SEM, 10 ormore primary particles were aggregated to form secondary particles inthe obtained active material powder for the positive electrode.

The initial discharge capacity at 25° C., 2.75 to 4.5 V, and a dischargerate of 0.5 C was 189.7 mAh/g, and the average voltage was 4.01 V. Thecapacity retention after 50 charge-discharge cycles was 93.2%. The heatgeneration starting temperature of the 4.3-V charged material was 174°C.

Example 3

An active material for a positive electrode was synthesized in the samemanner as in Example 1 except that silicon dioxide powder was used inplace of tungsten oxide, and composition analysis, property measurement,and battery performance test were conducted. As a result, thecomposition was LiCo_(0.99)Mg_(0.005)Si_(0.005)O₂.

The specific surface area of the powder after firing measured by anitrogen adsorption method was 0.36 m²/g, and the average particlediameter D50 measured by a laser scattering type particle sizedistribution analyzer was 15.2 μm. Magnesium and silicon were present onthe surface. As a result of observation through SEM, 10 or more primaryparticles were aggregated to form secondary particles in the obtainedactive material powder for the positive electrode.

The initial discharge capacity at 25° C., 2.75 to 4.5 V, and a dischargerate of 0.5 C was 190.7 mAh/g, and the average voltage was 3.99 V. Thecapacity retention after 50 charge-discharge cycles was 90.0%.

Comparative Example 1

An active material for a positive electrode was synthesized in the samemanner as in Example 1 except that magnesium hydroxide and tungstenoxide were not used, and composition analysis, property measurement, andbattery performance test were conducted. As a result, the compositionwas LiCoO₂.

The specific surface area of the powder after firing measured by anitrogen adsorption method was 0.32 m²/g, and the average particlediameter D50 measured by a laser scattering type particle sizedistribution analyzer was 13.3 μm.

The initial discharge capacity at 25° C., 2.75 to 4.5 V, and a dischargerate of 0.5 C was 194.5 mAh/g, and the average voltage was 4.01 V. Thecapacity retention after 50 charge-discharge cycles was 74.4%. The heatgeneration starting temperature of the 4.3-V charged product was 163° C.

Comparative Example 2

An active material for a positive electrode was synthesized in the samemanner as in Example 1 except that magnesium hydroxide was not used, andcomposition analysis, property measurement, and battery performance testwere conducted. As a result, the composition was LiCo_(0.99)W_(0.01)O₂.

The specific surface area of the powder after firing measured by anitrogen adsorption method was 0.66 m²/g, and the average particlediameter D50 measured by a laser scattering type particle sizedistribution analyzer was 13.81 μm. Tungsten was present on the surface.

The initial discharge capacity at 25° C., 2.75 to 4.5 V and a dischargerate of 0.5 C was 184.6 mAh/g, and the average voltage was 4.02 V. Thecapacity retention after 50 charge-discharge cycles was 80.7%.

In the same manner as in Example 1, X-ray diffraction spectra of thepowder after firing was obtained by a high-sensitivity X-ray diffractionmethod using Cu—Kα. The obtained spectrum is shown in FIG. 2. From FIG.2, diffraction spectrum was markedly observed within the range of 2θ=21±1°, and it was known that about 40% of tungsten is present as asingle oxide. It was also known from the analysis of the X-raydiffraction spectrum that about 50% of tungsten formed a solid solutionwith cobalt, and about 10% was present as Li₂WO₄.

Comparative Example 3

An active material for a positive electrode was synthesized in the samemanner as in Example 3 except that magnesium hydroxide was not used, andcomposition analysis, property measurement, and battery performance testwere conducted. As a result, the composition was LiCo_(0.99)Si_(0.01)O₂.

The specific surface area of the powder after firing measured by anitrogen adsorption method was 0.41 m²/g, and the average particlediameter D50 measured by a laser scattering type particle sizedistribution analyzer was 14.2 μm. Silicon was present on the surface.

The initial discharge capacity at 25° C., 2.75 to 4.5 V, and a dischargerate of 0.5 C was 193.3 mAh/g, and the average voltage was 4.01 V. Thecapacity retention after 50 charge-discharge cycles was 64.4%.

Comparative Example 4

An active material for a positive electrode was synthesized in the samemanner as in Example 1 except that tungsten oxide was not used, andcomposition analysis, property measurement, and battery performance testwere conducted. As a result, the composition was LiC_(0.99)Mg_(0.01)O₂.

The specific surface area of the powder after firing measured by anitrogen adsorption method was 0.29 m²/g, and the average particlediameter D50 measured by a laser scattering type particle sizedistribution analyzer was 13.3 μm. Magnesium was present on the surface.

The initial discharge capacity at 25° C., 2.75 to 4.5 V, and a dischargerate of 0.5 C was 190.1 mAh/g, and the average voltage was 3.980 V. Thecapacity retention after 50 charge-discharge cycles was 74.7%.

INDUSTRIAL APPLICABILITY

According to the present invention, as described above, an usefulpositive electrode material for a lithium secondary battery that hashigh cyclic durability and high safety at high-voltage and high-capacityusage is provided.

1. A positive electrode material for a lithium secondary battery,comprising an active material including particles, each of the particlescomprising a composition represented by a general formula,Li_(a)Co_(b)Mg_(c)A_(d)O_(e)F_(f), wherein the element A is tungsten orsilicon, 0.90≦a≦1.10, 0.97≦b≦1.00, 0.0001≦c≦0.03, 0.0001≦d≦0.03,1.98≦e≦2.02, 0≦f≦0.02 and 0.0001≦c+d≦0.03, and wherein the magnesium andthe element A, or further fluorine are evenly present in a vicinity of asurface of said particle.
 2. The positive electrode material for alithium secondary battery according to claim 1, wherein at least a partof the magnesium or the element represented by said A contained theparticles substitutes cobalt atom in said particles and forms a solidsolution.
 3. The positive electrode material for a lithium secondarybattery according to claim 1, wherein the atomic ratio of said magnesiumto said element A is 0.10≦c/d≦10.00, and 0.0002≦c+d≦0.02.
 4. Thepositive electrode material for a lithium secondary battery according toclaim 1, wherein said element A is silicon.
 5. The positive electrodematerial for a lithium secondary battery according to claim 1, wherein aquantity of the single oxide of said element A present in said activematerial for the positive electrode is 20% or less.
 6. The positiveelectrode material for a lithium secondary battery according to claim 1,wherein 10 or more particles are aggregated each other to form secondaryparticles, and an average particle diameter of said secondary particlesis 2 to 20 μm.
 7. The positive electrode material for a lithiumsecondary battery according to claim 1, wherein 0<f≦0.02.
 8. A positiveelectrode material for a lithium secondary battery wherein an activematerial for a positive electrode consists of a particle having acomposition represented by a general formula,Li_(a)Co_(b)Mg_(c)A_(d)O_(e)F_(f), wherein the element A is tungsten,0.90≦a≦1.10, 0.97≦b≦1.00, 0.0001≦c≦0.03, 0.0001≦d≦0.03, 1.98≦e≦2.02,0≦f≦0.02 and 0.0001 ≦c+d≦0.03, and wherein the magnesium and the elementA, or further fluorine are evenly present in a vicinity of a surface ofsaid particle, and no diffraction peaks are observed within a range of2θ=21±1° in the high-sensitivity X-ray diffraction spectrum using Cu—Kα.9. A method for manufacturing a positive electrode material for alithium secondary battery, comprising: preparing an active materialincluding primary particles, each of the particles comprising acomposition represented by a general formula,Li_(a)Co_(b)Mg_(c)A_(d)O_(e)F_(f), wherein the element A is tungsten orsilicon, 0.90≦a≦1.10, 0.97≦b≦1.00, 0.0001≦c≦0.03, 0.0001≦d≦0.03,1.98≦e≦2.02, 0≦f≦0.02 and 0.0001≦c+d≦0.03, and wherein the magnesium andthe element A, or further fluorine is evenly present in the vicinity ofa surface of said particles, and mixing a cobalt material containing atleast either cobalt oxyhydroxide or cobalt hydroxide, lithium carbonate,and a material containing magnesium, said element A or fluorine andfiring the mixture, wherein the positive electrode material comprisessecondary particles, each comprising 10 or more said primary particles.10. The method for manufacturing a positive electrode material accordingto claim 8, wherein 0<f≦0.02.